In known processes for microencapsulation, there are produced cores of encapsulated material surrounded by continuous shells of solid encapsulating material. These are usually performed by suspending the core material in a solvent and depositing the shell in liquid form from the solvent on the core. Liquid shell is then converted to a rigid solid shell. For example, many solids have been encapsulated with ethylcellulose by suspension of the solids in a cyclohexane solution of ethylcellulose containing another polymer such as polyethylene at a temperature of 80.degree. C. Cooling the solution forms minute droplets of ethylcellulose which enveloped the solid particles. Further cooling gives capsules of minute size having the solid encapsulated in ethyhlcellulose, and these can be recovered and dried by methods known in the art.
There are two broad types of microencapsulated products, in terms of particle size of the internal phase (the material being encapsulated): 1. Relatively large materials, 250 microns across, or even as small as 75 to 100 microns, are usually enrobed individually. 2. Very small materials, less than a micron to 10 or 20 microns, are usually enrobed as aggregates. That is, the enrobing polymer gathers scores or hundreds of particles within a single microcapsule.
Examination of microcapsules of the first type at as low as 40X magnification shows a clearly defined wall, of 2 to 50 microns, depending on the amount of polymer used. Examination of microcapsules of the second type at as high as 1,000X magnification shows a barely discernible wall, in some cases indiscernible. Such walls range from 1 or 2 microns thick, down to hundredths or thousandths of a micron.
The distinguishing feature between microcapsules of type 1 and type 2 is the distribution of wall or polymer material. In the first type, the wall material is distributed on the periphery of single particles. In the second type the polymer is distributed throughout the microcapsule, as well as on the periphery of the capsule. The tremendous surface area within the microcapsule, provided by the aggregation of minute particles, leaves very little polymer for the periphery.
One of the disadvantages of the prior art is that it is difficult to control the size of the microcapsule since the stirring rate needed to disperse the solid core causes the formation of very small capsules. This invention, where it concerns particle size control, has to do with microcapsules of the second type.
The rates of stirring required to disperse the internal phase are such that very small capsules generally result. There is an equilibrium of aggregation and dispersion of the material being stirred and distributed in the solvent. This equilibrium is pushed in the direction of dispersion due to the required high stirring rate. The rate that is thus required will yield correspondingly small microcapsules.
Another disadvantage of the prior art is that it is difficult to obtain good bioavailability of moderately water soluble solids. There is no problem in respect to bioavailability of highly water soluble internal phases. Gastric or intestinal fluid diffuses through the microcapsule wall and dissolves internal phase material. The resultant solution diffuses from the microcapsule to the gastrointestinal tract. The diffusion in each direction continues over a period of time until essentially all of the internal phase is released. Sparingly soluble substances present a problem in this context, since only small quantities of dissolved material diffuse from the microcapsule into the gastrointestinal tract.
A still further disadvantage of the known method is that productivity of microcapsules per batch is limited by the amount of solids that can be dissolved and dispersed in the continuous liquid phase in which the preparation takes place.